FIG. 1 schematically shows a cross-section of two adjacent photosensitive cells 10, 12 of a conventional CMOS-type image sensor formed on a substrate 13. Such a sensor corresponds, for example, to the sensor sold by STMicroelectronics under the trade name “CMOS Image Sensor Module VS6552”. Each photosensitive cell 10, 12 is associated with a portion of the surface of substrate 13 which, in a top view, generally has the shape of a square or of a rectangle. Each photosensitive cell 10, 12 includes an active photosensitive area 14, 16, generally corresponding to a photodiode adapted to storing a quantity of electric charges according to the received light intensity. Substrate 13 is covered with a stacking of insulating and transparent layers 18, for example, formed of silicon oxide. Conductive tracks 20, formed on the surface of substrate 13 and between adjacent insulating layers, and conductive vias 22, formed through insulating layers 18, especially enable addressing photosensitive areas 14, 16 and collecting electric signals provided by photosensitive areas 14, 16. Conductive tracks 20 and conductive vias 22 are generally formed of reflective or absorbing materials. In a color sensor, a color filter element, for example, an organic filter 24, 26, is arranged at the surface of the stacking of insulating layers 18 at the level of each photosensitive cell 10, 12. The elements of color filter 24, 26 are generally covered with a planarized equalizing layer 27 which defines an exposition surface 28 exposed to light.
Photosensitive area 14, 16 generally does not cover the entire surface of substrate 13 associated with photosensitive cell 10, 12. Indeed, a portion of the surface is reserved to devices for addressing and reading from photosensitive area 14. A photosensitive area 14 generally covers approximately 30% of the surface of substrate 13 associated with photosensitive cell 10, 12. To increase the light intensity reaching the photosensitive area of a photosensitive cell, a microlens 29, 30 is arranged on equalizing layer 27, opposite to photosensitive area 14, 16 to focus the light beams towards photosensitive area 14, 16. The paths followed by three light beams R1, R2, R3 are schematically shown as an example in stripe-dot lines for photosensitive cells 10, 12. Conductive tracks 20 and conductive vias 22 are arranged to avoid hindering the passing of the light beams.
Microlenses 29, 30 are generally obtained by covering equalizing layer 27 with a resin, etching the resin to define separate resin blocks, each resin block being formed substantially opposite to a photosensitive area 14, 16, by heating the resin blocks. Each resin block then tends to deform by reflow, the center of the block inflating and the lateral walls collapsing, to obtain a convex external surface 32, 34. The external surface 32, 34 desired to ensure an optimal focusing of the light beams towards a photosensitive area corresponds to a portion of a sphere having its radius varying proportionally to the distance separating a microlens 29, 30 from the associated photosensitive area 14, 16. As an example, for a photosensitive cell 10, 12 with a 4-micrometer side and for a distance on the order of from 8 to 10 micrometers between a microlens 29, 30 and the associated photosensitive area 14, 16, the maximum thickness of the microlens 29, 30 is approximately ½ micrometer.
The previously-described method of manufacturing microlenses 29, 30, however, does not enable obtaining a microlens 29, 30 filling the entire portion of the exposition surface associated with the photosensitive cells. Indeed, the resin blocks from which microlenses 29, 30 are formed must be separated from one another by separation regions 36 surrounding each resin block, the minimum width of which especially depends on the used etch techniques and on the used resin type. For conventional etch techniques, separation regions 36 have a minimum width from approximately 0.4 to 0.5 micrometer, which substantially corresponds to 10% of the side of a photosensitive cell. Separation regions 36 are maintained after forming microlenses 29, 30. A circular resin block enables obtaining a microlens 29, 30 having an external surface substantially corresponding to a spherical portion. However, to reduce separation regions 36 to a minimum while keeping an external microlens surface relatively close to a spherical portion, a resin block having, as seen from above, the shape of a square or of a rectangle with tapered angles, is generally used. The light arriving at the level of separation regions 36 associated with a photosensitive cell is not focused towards photosensitive area 14, 16, which reduces the sensor's sensitivity.
A solution to increase the light intensity focused towards the photosensitive area of a photosensitive cell is to provide an additional so-called “top-coating” step, which includes the conformal deposition of a transparent material (not shown), for example, silicon nitride, on microlenses 29, 30. The external surface of the conformal deposition follows the shape of microlenses 29, 30 and forms the light-focusing surface. The conformal deposition then provides a focusing surface including dished areas at the level of each microlens 29, 30. Two adjacent dished areas are separated by a minimum distance less than the minimum width of the separation region between the two associated microlenses. When the conformal deposition has a sufficient thickness, the dished surfaces can be contiguous.
To increase the sensitivity of an image sensor, it is desirable to increase the number of photosensitive cells forming it. However, it is not desirable for the total surface area taken up by the sensor to excessively increase. It is thus desirable to decrease the surface area of a photosensitive cell. This imposes decreasing the surface area of the photosensitive area of each photosensitive cell. The sensitivity of each photosensitive cell is decreased since the photosensitive area of the photosensitive cell receives a lower and lower light intensity. The optimizing of the amount of light received by the photosensitive area of a photosensitive cell with respect to the amount of light received by the portion of the exposition surface associated with the photosensitive cell then becomes an important factor.
The performing of a conformal deposition increases the distance between each dished area and the associated photosensitive area. The more distant a dished area is from the associated photosensitive area, the higher its radius of curvature must be to ensure a proper focusing of the light beams towards the photosensitive area. This requires the forming of a microlens, itself having a high radius of curvature. The radius of curvature of a microlens is inversely proportional to the thickness of the resin block from which the microlens originates. However, the lower the thickness of a resin block, the more difficult it is to accurately control the radius of curvature of the finally-obtained microlens.
Furthermore, at small scales, it is difficult to form a perfectly conformal deposition and thus ensure for the external surface of the conformal deposition to accurately follow the convex surface of the microlenses.